Re: [ATM] 7 inch sphereical mirror

["Richard F.L.R. Snashall" <rflrs@rcn.com>]

Dominic-Luc Webb wrote:
>
> Did I miss something? My understanding was that coma is the
> price paid when you make a parabola. Spherical aberration is
> what you get with a sphere. At some point, the trade-off
> between these two defects should be considered. Coma increases
> as one progresses off axis. I think an F/10 can perform very
> well. This scope is in the range of a lensless Schmidt, and
> there are some very impressive images out there of objects like
> Andromeda galaxy covering something like 3 degrees of sky.
At 21 mm off-axis (0.6767 degrees), I get the Visual Strehl of
about 0.34 and at the 70.7% point of that, about 0.58.  The
"diffraction limited" field (0.80) is about 0.5 degree diameter.

Of course the field is convex to the viewer, so there may be
additional losses due mismatch with the eyepiece.


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